THE USE OF PECVD SILICON OXYNITRIDE THIN-FILMS IN MICROELECTRONICS MANUFACTURING

Abstract

In the semiconductor manufacturing, the high level of reflection from the substrates becomes intolerable in sub-0.25-llm optical lithography by using KrF excimer laser stepper. Linewidth variation and notching are commonly observed during microlithographic masking. Consequently, unacceptable critical dimension control, micro trenches and poor electrical characteristics are obtained after etching. In order to reduce the effects of substrate reflection on the photoresists and achieve a high CD (Critical Dimension) controllability, a high performance Bottom Anti-Reflective Coating (BARC) is necessary for sub-0.25-llm optical lithography. In this study, a novel bottom antireflective method is described to resolve the problems with conventional ones, such as insufficient antireflection, substrate material and structure dependence. The antireflection layer with high photo absorption at the bottom absorbs most of light reflected from the substrate and the interference layer at the top suppresses the surface reflection of this layer. This new method can achieve extremely low substrate reflectivity (0.5%) and is very effective for all substrate and film stacks. In addition, it can provide larger variation on film thickness compared with the conventional methods. In this project, the silicon oxynitride film is chosen as the BARC material. To obtain a wide variation of film optical property, a new method to fabricate the films is proposed. The new method that has been developed in this project utilizes SiH4, N₂, N₂O and He as reactant gases to deposit silicon oxynitrides by plasma enhanced chemical vapor deposition (PECVD). Comparing with the conventional method using SiH₄ and N₂O as reactant gases, it provides a much wider range of film compositions that can meet many requirements. In addition, lower concentrations of hydrogen are incorporated into the films. To produce high quality oxynitrides and increase the probability of the reaction between SiH₄ and N₂, some important process parameters are investigated to find an optimum working point. The refractive index n and the extinction coefficient k values are correlated to the SiOxNʸ stoichiometry. The n value forms a good linear relationship with the concentration ratio of O/(O+N) while the k value is determined by the silicon content in the films. Lastly, in order to investigate the applicability of PECVD-deposited silicon oxynitrides for the LOCOS (Local Oxidation of Silicon) process, wet oxidation of these films is studied for layer compositions ranging from that of pure nitride to oxynitride with an atomic ratio of [O]/([O]+[N])=0.5. Near-stoichiometric and silicon- rich silicon oxynitrides have been studied. The results obtained by Denisse (72) are confirmed: (i) fast oxidation in silicon oxynitride if hydrogen is mainly bonded to nitrogen, and (ii) slower oxidation if the silicon oxynitride contains higher Si-H bonds because it can lose its hydrogen via cross-linking before it is oxidized. The oxidation rate for silicon-rich silicon oxynitride at [O]/([O]+[N]) < 0.4 is nearly one order of magnitude smaller than that of near-stoichiometric oxynitrides and nearly two orders of magnitude smaller than that of Si. It may be postulated from XPS depth profiling that the fundamental mechanism of SiOxNʸ oxidation is by progressive O-for-N substitution in the silicon oxynitride tetrahedron unit, which is best designated as SiN₂. xO2+x˖ where x is also an index of depth. In addition to the higher Si-H content, the more densified structure of silicon rich silicon oxynitride after high temperature oxidation may also be responsible for the high oxidation resistance of the films, since it limited the substitution of O for N, and then suppress the oxidation rate of the silicon rich silicon oxynitrides.